How to sew a turtleneck/cowl neckline
Quick and Easy, good for one-off cowl turtlenecks:
Sew up the tee shirt pattern as instructed, leaving out the neckline binding. If you're using Travel Trio One, omit neckline binding. If you're using Three Tees, do not finish the neckline.
To make a deeper cowl, you'll want to have the neckline lower than the typical tee shirt. Remove 1" from the neckline all the way around. Continue to the next step
Hack an existing pattern for a cowl turtleneck, good if you want to re-make this turtle/cowl again:
Using your flat pattern or a traced copy, Measure 1.5" below the neckline at the CF fold, the shoulder and halfway between. Connect these lines in a curve mirroring the neckline shape. Repeat for the back neckline.
Cut the pattern out 1/2" ABOVE the line you just marked (so you'll have a 1/2" Seam Allowance for the cowl.) You'll trim off the top of the pattern piece at the neckline, so make a copy of the pattern pieces first.
For both methods:
Measure the new neckline circumference after your tee is sewn, but BEFORE you finish your neckline
Make a cowl/turtleneck pattern, using your measurement from Step 1 X 18". We like 18" because it's nice and deep, and will make a 9" folded cowl when finished. You can adjust this if you wish - go up to 12" for a super draped cowl, especially in a soft knit. Make sure the stretch is going crosswise along the neckline measurement when you are cutting the fabric. You want the stretch to go around your neck (not up and down.)
Fold cowl/ turtleneck in half, right sides together, pin and sew back seam
Turn the cowl/turtle out and fold in half, WRONG sides together, matching up the bottom raw edges and enclosing the back seam. Try on your cowl to see how you like the fit and depth of it.
Edge serge the raw edges together
Line up the turtleneck back seam with the center back neckline of your shirt
Pin around, pinning the raw edge of the tee and the edge serged edge of the cowl/turtleneck together.
Sew the cowl / turtleneck on to the tee using a serger, zigzag, or stretch stitch. Press seam down and edge stitch the seam allowance down with a long straight stitch to secure seam in place.